1. Field of the Invention
This invention relates to the so-called light recording medium which reads information given as a physical deformation on a recording layer.
2. Description of the Prior Art
What have heretofore been employed as media for recording information are magnetic recording media, but they have several disadvantages, for example, the information recording density is limited, there is also a limitation on improvement of noise characteristics, the medium is not free from abrasion since it is used in the state where the medium is in intimate contact with a head, and so forth. For that reason, light recording media have recently come to attract attention as the substitute for the magnetic recording media.
Among the light recording media, the so-called heat mode recording which utilizes light as heat involves selectively irradiating a medium with coherent light such as laser light to melt or burn the medium to provide a small hole thereon, and recording information by the presence or absence of said small hole. As materials used for the prior art heat mode recording, tellurium, tellurium-selenium-arsenic compounds, mixtures of nitrocellulose and light absorbers etc. are known.
In the prior art heat mode recording, as illustrated in FIG. 1, a laser beam 16 is condensed on a recording layer 12 through a lens 10, and irradiates the recording layer 12 as a minute spot of 1 .mu.m.phi. or so. The irradiated area on the recording layer attains a high temperature in a short time of about 100 mS. Where the medium is a nitrocellulosic material, when the irradiated area is heated to 180.degree. C. or higher, said area is burnt and leaves a small hole of about 1 .mu.m as shown as 18 in FIG. 2, and by this hole 18, information of 1 bit is recorded. In FIG. 1 and FIG. 2, 14 is a substrate for supporting the recording layer 12. On the other hand, where the medium is based on tellurium, as shown in FIG. 3, a recording layer 12a melts due to a high temperature and forms a small hole 18a by the surface tension. Since the melting temperature of tellurium is relatively high, the recording layer 12a must be extremely thin in order to ensure the necessary high temperature, and a film thickness of about 50-100 nm is generally chosen.
However, in the prior art light recording as described above, the physical deformation imparted to the medium for recording is permanent, and therefore it has a disadvantage that the record is not erasable and hence the medium is not reusable. So when compared with the prior art magnetic recording system which permits erasing and rewriting of information, the light recording system which does not permit rewriting is greatly restricted in application.
On the other hand, there has heretofore been known a recording system as a recording medium used in photography, and this system involves imparting electron charges to a medium of a thermoplastic material and deforming the medium by attraction force between a separately provided electrode and the electron charges, and by this system, the record may be erased by heating the medium. However, this system has disadvantages that the mechanism to impart electron charges to a medium is complicated and also that electricity consumption is great.
Furthermore, there are also known photochromic materials and chalcogenite materials as the prior art erasble light recording media, but the former has a disadvantage that the record is unstable and hence information spontaneously disappears, and the latter has a disadvantage that they are harmful to the human body.